1. Field of the Invention
The invention relates to internal combustion engines which include variable cam timing systems, wherein a pressurized flow of valvetrain-lubricating oil to the cylinder head is controllably diverted to operate a cam phaser unit, for example, mounted on an end of a cam shaft.
2. Background Art
The prior art teaches internal combustion engines wherein a fixed relationship between camshaft rotation and crankshaft rotation is maintained to thereby preserve the relationship between intake and exhaust valve events and piston motion. Alternatively, the prior art teaches so-called variable cam timing engines which seek to adjust this relationship to achieve such advantages as increased fuel economy and reduced regulated emissions. Under one prior art approach, as disclosed in U.S. Pat. No. 5,363,817, actual cam timing is measured using a toothed wheel on the camshaft and a toothed wheel on the crankshaft. The time, or angle, between receiving pulses from the wheel on the crankshaft and the wheel on the camshaft represents the actual cam timing. A desired cam timing is determined as a function of engine operating conditions, and an error signal is created from the difference of the desired cam timing and the actual cam timing. Control signals based upon the error signal are then generated and supplied to actuators capable of adjusting the cam timing, typically by supplying pressurized oil to a solenoid valve that controllably diverts the pressurized oil to a selected passage in the camshaft which, in turn, directs the pressurized oil to an oil-pressure-responsive cam phaser unit.
Significantly, in such a system, the cylinder heads are specifically designed for variable cam timing, i.e., the heads include oil-porting passages to communicate pressurized oil to "advance" and "retard" passages defined in the camshaft. Such systems thus require extensive design considerations and machining of the heads and camshaft to accommodate the oil passages, as well as consideration to the mounting of the solenoid valves.
Alternatively, the prior art teaches use of ported camshaft bearings to define the paths by which pressurized oil is supplied to a camshaft-mounted cam phaser unit. Such ported camshaft bearings either require similar oil-porting passages in the cylinder head, or external oil routing tubes and flow control structures which, in turn, present additional packaging and mechanical attachment issues, each serving to increase the cost of providing variable cam timing to an existing engine. Such external oil routing tubes and flow control structures also constitute additional parts to be installed or assembled to the engine, thereby serving to further increase engine assembly space and manpower requirements.